Sorry. I wasn't aware that you were unsure about ARF PRSI being Class S. The P60 that used to be issued by the ARF providers indicated this.
My reading of it is that you need 52 Class S contributions in the governing year. 39 only seems to refer to A, H or P contributions (or credits) for that year. (Of course you have to meet the other minimum PRSI criteria also).
I am aware of the post you to which you linked suggesting that you could maintain a full Class S record with a drawdown of less than €12,500 if the drawdowns are scheduled monthly. Perhaps, but I would be reluctant to rely on it without further verification.
For one reason, I noted 52 Class S on my record one year (in addition to A) even though the total drawdown was much less than €12,500. However, on a more recent update of the record there is just one Class S. I didn't check this further because it didn't impact on me, as I had the Class A anyway.
A second reason is the minimum voluntary payment of €500. I would be surprised if you could get a full record by paying less than this by way of an ARF. I recall another thread where a poster suggested than an ARF PRSI payment of less than €500 could be topped by way of voluntary contribution to the required €500 for the purpose of maintaining a full record (I haven't searched for the thread).